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Other Terms: Sphenoidal bone, Sphenoid, Os sphenoidale
The sphenoid is a complex bone taking on the spread winged appearance of a butterfly. It is divisible into four principal components: body, greater wings, lesser wings, and pterygoid processes. It forms the central base of the skull. With the cranial cap removed the bone is visible from any view. This bone plays a prominent role at the base of the skull. It supports the brain, serves to protect the optic stalks and capsules, provides passage for many vessels and nerves entering and leaving the skull. The body of the bone is cavitated forming an osseous sinus that communicates with the nasal cavity.
Sphenoid is from the Greek sphene or wedge and eidos suggesting shape. This then is a wedge-like bone, probably from its central wedge-like location in the middle of the skull. There are, seemingly erroneous, suggestions that it was derived from the Greek word sphecoid, only later becoming sphenoid through mistranslation. Spheco means wasp, from the suggestion that its wing-like shape resembled a wasp. The term sphenoid, however, is traceable to the first century writings of Claudius Galen. No earlier evidence of it or sphecoid exists.
The sphenoid articulates with a total of twelve other bones: the vomer, the ethmoid, the frontal, and the occipital, each single bones; additionally, it articulates with four paired bones, the parietal, temporal, zygomatic, and palatine.
The sphenoid is not only a complex bone in its adult form and function, but also has a complex ossification history that reflects its evolutionary past. It receives contributions from the chondrocranium, splanchnocranium, and dermatocranium. During the embryonic period the bone is pre-formed in cartilage. This cartilage primordium then begins to ossify from fourteen centers throughout the fetal period. During most of this period it is divisible into two parts: the pre-sphenoid consisting of the anterior portion of the body along with the lesser wings and a post-sphenoid comprising the posterior body with the attached greater wings and pterygoid processes. There are six ossification centers for the pre-sphenoid and eight ossification centers for the post-sphenoid. During the ninth week of fetal life, the first two pre-sphenoidal centers appear just lateral to the optic canal at the base of the lesser wings (orbitosphenoids). In the tenth week two centers emerge in the pre-sphenoidal body. The final centers arise during the fifth fetal month as the sphenoidal conchae. These final two are the slowest centers to develop, not completing ossification and joining the rest of the sphenoid until the twelfth postnatal year. The first centers for the post-sphenoid arise during the eighth embryonic week. This center, between the foramen rotundum and foramen ovale, forms the site for the proximal portion of the greater wing (alisphenoids). Intramembranous ossification forms the larger part of the greater wing. In the ninth week, two centers emerge at the sides of the sella turcica or body of the post-sphenoid. The hamulus of the pterygoid plates each begin to ossify during the third fetal month. The pterygoid lingulae emerge as centers of ossification during the fourth month. Most of the pterygoid processes form from intramembranous ossification centers. By birth or shortly after most of the centers have united.